The present invention relates to the sealing of relatively movable parts and to the sealing of relatively movable parts within pumping apparatus, particularly (but not exclusively) pumping apparatus for viscous fluids which are sensitive to shearing forces within the fluid.
A number of different types of pumps are used to pump liquids. However, problems can occur with the pumping of viscous fluids which are prone to set if subjected to high frictional forces.
One such viscous fluid is UV ink, which is an ink of a high viscosity which cures when subjected to ultra-violet radiation. The properties of the UV ink which enable it to set when it is irradiated unfortunately render it very sensitive to friction. Viscous fluids require the build up of a high pressure in order for them to be pumped.
One pump which is known to provide very little frictional force on the material being pumped is a double-diaphragm pump. Double-diaphragm pumps do not have a sliding surface between which the material to be pumped travels and therefore there is little chance of a high frictional force being generated which might cure the material. However, due to the inherent limitation of the flexible diaphragm material these pumps are restricted to low pressure applications, namely a maximum of about 13.6 atmospheres (200 Psi). Thus, such pumps are not suitable for pumping material of a high viscosity.
Other known pumps include single and double-acting pumps which have a piston which is reciprocated between an inlet and an outlet such that fluid is forced from below the piston to above the piston during a pumping cycle. In one type of double-acting pump fluid is forced between two separate chambers in the pump and a valve between the chambers allows the fluid to be forced through small apertures which open between the two chambers during displacement of the piston. In another type of pump the piston itself has several small apertures through which the fluid is forced, during the reciprocating movement of the piston, in order to move fluid from the inlet to the outlet. When reciprocated in the opposite direction, the piston is now required to force the fluid that has been passed through its small apertures out of the second chamber through the outlet of the pump. A valve is used to seal the piston apertures but the piston itself has to be slidingly sealed to the chamber wall.
Each of the above pumps therefore has the drawback that the small clearances and the meshing of several opposing surfaces can produce a high frictional force on the fluid being pumped which cause shear sensitive materials such as UV ink to cure within the pump and consequently cause the pump to fail.
It is essential in the single and double-acting pumps for there to be a very efficient seal between the piston and the cylinder within which it is disposed. If fluid is allowed to pass the seal, it is subjected to friction which causes the ink to set and which normally causes very rapid seizure of the pump. It is possible to design seals which provide the necessary high degree of sealing, but they are relatively expensive and require skilled maintenance from time to time.
It is an object of the present invention to provide a sealing arrangement between relatively movable parts which overcomes or alleviates the problems of the prior art.
In accordance with a first aspect of the present invention, in an assembly comprising an elongate member slidably disposed with respect to a sealing member, the elongate member comprises a resiliently deformable sleeve or coating and the sealing member comprises a substantially rigid sealing portion which engages the sleeve or coating and resiliently deforms it in the region of contact.
By providing an elongate resilient area, different portions of the resilient sleeve or coating are frictionally engaged as the elongate member moves. This is in contrast to the prior art having a fixed, resilient seal, which is continuously and frictionally engaged as the elongate member moves.
The sleeve or coating may comprise a single layer, for example polyurethane. Alternatively, the sleeve or coating may comprise a plurality of layers. For example, the sleeve or coating may comprise a resilient inner sleeve (of polyurethane) and an outer sleeve of a different material (e.g. PTFE).
The sleeve or coating need only be provided in that region of the elongate member which will frictionally engage the substantially rigid sealing portion.
In accordance with a second aspect of the present invention, in an assembly comprising a piston slidably disposed within a cylinder, the cylinder comprises a resiliently deformable lining and the piston comprises a substantially rigid sealing portion which engages the lining and resiliently deforms it in the region of contact.
Such an arrangement ensures that different parts of the lining are frictionally engaged by the piston as the piston moves. This is in contrast to the prior art in which a resilient seal on the piston is in continuous frictional engagement with the cylinder wall.
The lining may comprise a single layer (e.g. polyurethane). Alternatively, the lining may comprise a plurality of layers. For example, the lining may comprise an outer, resiliently deformable layer (e.g. polyurethane) and an inner layer (e.g. PTFE).
In accordance with a third aspect of the present invention, a fluid pump comprises a housing having an internal displacement chamber, an inlet, an outlet and a displacement rod which passes reciprocally into the displacement chamber through an aperture in the wall of the pump housing, wherein the displacement rod comprises a resiliently deformable sleeve or coating and the housing comprises a sealing member having a substantially rigid sealing portion which engages the sleeve or coating and resiliently deforms it in the region of contact
In accordance with a fourth aspect of the present invention, a fluid pump comprises a housing having an internal displacement chamber, an inlet, an outlet and a displacement rod which is reciprocable sealingly within the displacement chamber, the displacement chamber comprising a resiliently deformable lining and the piston comprising a substantially rigid sealing portion which engages the lining and resiliently deforms it in the region of contact.
In all of the above aspects, the substantially rigid sealing portion is preferably annular and the resiliently deformable portion with which the substantially rigid sealing portion is engageable is tubular.
In accordance with a fifth aspect of the present invention, a fluid pump comprises a housing having an internal displacement chamber, an inlet, an outlet and a displacement rod which passes reciprocally into the displacement chamber through an aperture in the wall of the pump housing, the pump further comprising inner and outer spaced apart seal means which engage the outer surface of the displacement rod and means for applying fluid to the surface of that portion of the displacement rod between the inner and outer seal means.
In one embodiment, the means for applying fluid comprises an annular piston carried by the displacement rod, the piston having a sealing surface of non-constant profile which is releasably engageable with a seal means to isolate a volume of fluid to be applied and which is movable to compress the isolated volume of fluid and thereby displace it towards the displacement rod. Preferably, the piston comprises a first portion of smaller diameter which is releasably engageable with the seal means and a second portion of larger diameter, whereby movement of the larger diameter portion towards the seal means reduces the isolated volume and thereby compresses the fluid to be applied to the rod.
There may also be pressure means to assist in the application of the fluid to the surface of the rod. This may comprise pressurised gas which causes the fluid to be sprayed onto the surface of the displacement rod. This may conveniently be obtained from the exhaust of an air actuator used for displacing the displacement rod.
Preferably, the displacement rod comprises a resiliently deformable sleeve or coating and the inner and outer seal means are substantially rigid.
In accordance with a sixth aspect of the present invention, a fluid pump comprises a housing having an internal displacement chamber, an inlet, an outlet, a displacement rod which passes reciprocally into the displacement chamber through an aperture in the wall of the pump housing and seal means which engage the outer surface of the displacement rod and define a boundary of the displacement chamber, the pump further comprising guide means to ensure that fluid displaced by the displacement rod passes over the inner portion of the seal means before being discharged through the outlet.
Such a construction ensures that the inner face of the seal is xe2x80x9cflushedxe2x80x9d during each stroke of the displacement rod, which helps to avoid build-up of deposits on the rod which might otherwise lead to pump failure.
The pump may comprise a tube through which the displacement rod passes with clearance and the tube defining one or more gaps adjacent to the seal means, forming the guide means for passage of pumped fluid but otherwise isolating the internal displacement chamber from the outlet.
One end of the tube may comprise a plurality of projections which space the end of the tube from the seal means, the gaps being defined between the projections. The projections may abut the seal means, in which case the gaps are partially defined by the seal means.
The other end of the tube may comprise an annular portion which engages the outer surface of the displacement rod. This may act as a scraper to discourage the build-up of unwanted deposits on the rod. The tube may comprise one or more apertures which allow passage into the tube of fluid to be pumped when the displacement rod is actuated.
In accordance with a seventh aspect of the present invention, a fluid pump comprises a housing having an internal displacement chamber, an inlet, an outlet, a displacement rod which passes reciprocally into the displacement chamber and seal means which engage the outer surface of the displacement rod and define a boundary of the displacement chamber, the displacement rod comprising a resiliently deformable sleeve or coating and the seal means being substantially rigid, the pump further comprising a scraper means outwardly of the seal means which comprises one or more edges for scraping unwanted deposits off the resiliently deformable sleeve or coating of the displacement rod.
Preferably, the scraper means comprises a tube having a plurality of apertures therein, the periphery of the apertures being formed into scraper edges.
Preferably, the diameter of the tube is smaller than the outer diameter of the displacement rod. This results in the resiliently deformable sleeve or casing bulging into the apertures, which enhances removal of unwanted deposits.